c-Fos proteasomal degradation is activated by a default mechanism, and its regulation by NAD(P)H:quinone oxidoreductase 1 determines c-Fos serum response kinetics

Protein arginine methyltransferases (PRMTs): Orchestrators of cancer pathogenesis, immunotherapy dynamics, and drug resistance

Protein Arginine Methyltransferases (PRMTs) are a family of enzymes regulating protein arginine methylation, which is a post-translational modification crucial for various cellular processes. Recent studies have highlighted the mechanistic role of PRMTs in cancer pathogenesis, immunotherapy, and drug resistance. PRMTs are involved in diverse oncogenic processes, including cell proliferation, apoptosis, and metastasis. They exert their effects by methylation of histones, transcription factors, and other regulatory proteins, resulting in altered gene expression patterns. PRMT-mediated histone methylation can lead to aberrant chromatin remodeling and epigenetic changes that drive oncogenesis. Additionally, PRMTs can directly interact with key signaling pathways involved in cancer progression, such as the PI3K/Akt and MAPK pathways, thereby modulating cell survival and proliferation. In the context of cancer immunotherapy, PRMTs have emerged as critical regulators of immune responses. They modulate immune checkpoint molecules, including programmed cell death protein 1 (PD-1), through arginine methylation. Drug resistance is a significant challenge in cancer treatment, and PRMTs have been implicated in this phenomenon. PRMTs can contribute to drug resistance through multiple mechanisms, including the epigenetic regulation of drug efflux pumps, altered DNA damage repair, and modulation of cell survival pathways. In conclusion, PRMTs play critical roles in cancer pathogenesis, immunotherapy, and drug resistance. In this overview, we have endeavored to illuminate the mechanistic intricacies of PRMT-mediated processes. Shedding light on these aspects will offer valuable insights into the fundamental biology of cancer and establish PRMTs as promising therapeutic targets.

Protection of c-Fos from autophagic degradation by PRMT1-mediated methylation fosters gastric tumorigenesis

Both AP-1 and PRMT1 are vital molecules in variety of cellular progresssion, but the interaction between these proteins in the context of cellular functions is less clear. Gastric cancer (GC) is one of the pernicious diseases worldwide. An in-depth understanding of the molecular mode of action underlying gastric tumorigenesis is still elusive. In this study, we found that PRMT1 directly interacts with c-Fos and enhances AP-1 activation. PRMT1-mediated arginine methylation (mono- and dimethylation) of c-Fos synergistically enhances c-Fos-mediated AP-1 liveliness and consequently increases c-Fos protein stabilization. Consistent with this finding, PRMT1 knockdown decreases the protein level of c-Fos. We discovered that the c-Fos protein undergoes autophagic degradation and found that PRMT1-mediated methylation at R287 protects c-Fos from autophagosomal degradation and is linked to clinicopathologic variables as well as prognosis in stomach tumor. Together, our data demonstrate that PRMT1-mediated c-Fos protein stabilization promotes gastric tumorigenesis. We contend that targeting this modification could constitute a new therapeutic strategy in gastric cancer.

NQO1 regulates cell cycle progression at the G2/M phase

Rationale: Overexpression of NAD(P)H:quinone oxidoreductase 1 (NQO1) is associated with tumor cell proliferation and growth in several human cancer types. However, the molecular mechanisms underlying the activity of NQO1 in cell cycle progression are currently unclear. Here, we report a novel function of NQO1 in modulation of the cell cycle regulator, cyclin-dependent kinase subunit-1 (CKS1), at the G2/M phase through effects on the stability of c‑Fos. Methods: The roles of the NQO1/c-Fos/CKS1 signaling pathway in cell cycle progression were analyzed in cancer cells using synchronization of the cell cycle and flow cytometry. The mechanisms underlying NQO1/c-Fos/CKS1-mediated regulation of cell cycle progression in cancer cells were studied using siRNA approaches, overexpression systems, reporter assays, co-immunoprecipitation, pull-down assays, microarray analysis, and CDK1 kinase assays. In addition, publicly available data sets and immunohistochemistry were used to investigate the correlation between NQO1 expression levels and clinicopathological features in cancer patients. Results: Our results suggest that NQO1 directly interacts with the unstructured DNA-binding domain of c-Fos, which has been implicated in cancer proliferation, differentiation, and development as well as patient survival, and inhibits its proteasome-mediated degradation, thereby inducing CKS1 expression and regulation of cell cycle progression at the G2/M phase. Notably, a NQO1 deficiency in human cancer cell lines led to suppression of c-Fos-mediated CKS1 expression and cell cycle progression. Consistent with this, high NQO1 expression was correlated with increased CKS1 and poor prognosis in cancer patients. Conclusions: Collectively, our results support a novel regulatory role of NQO1 in the mechanism of cell cycle progression at the G2/M phase in cancer through effects on c‑Fos/CKS1 signaling.

Neoplasia-associated Chromosome Translocations Resulting in Gene Truncation

Chromosomal translocations in cancer as well as benign neoplasias typically lead to the formation of fusion genes. Such genes may encode chimeric proteins when two protein-coding regions fuse in-frame, or they may result in deregulation of genes via promoter swapping or translocation of the gene into the vicinity of a highly active regulatory element. A less studied consequence of chromosomal translocations is the fusion of two breakpoint genes resulting in an out-of-frame chimera. The breaks then occur in one or both protein-coding regions forming a stop codon in the chimeric transcript shortly after the fusion point. Though the latter genetic events and mechanisms at first awoke little research interest, careful investigations have established them as neither rare nor inconsequential. In the present work, we review and discuss the truncation of genes in neoplastic cells resulting from chromosomal rearrangements, especially from seemingly balanced translocations.

Targeting HIF-1α Function in Cancer through the Chaperone Action of NQO1: Implications of Genetic Diversity of NQO1

HIF-1α is a master regulator of oxygen homeostasis involved in different stages of cancer development. Thus, HIF-1α inhibition represents an interesting target for anti-cancer therapy. It was recently shown that the HIF-1α interaction with NQO1 inhibits proteasomal degradation of the former, thus suggesting that targeting the stability and/or function of NQO1 could lead to the destabilization of HIF-1α as a therapeutic approach. Since the molecular interactions of NQO1 with HIF-1α are beginning to be unraveled, in this review we discuss: (1) Structure–function relationships of HIF-1α; (2) our current knowledge on the intracellular functions and stability of NQO1; (3) the pharmacological modulation of NQO1 by small ligands regarding function and stability; (4) the potential effects of genetic variability of NQO1 in HIF-1α levels and function; (5) the molecular determinants of NQO1 as a chaperone of many different proteins including cancer-associated factors such as HIF-1α, p53 and p73α. This knowledge is then further discussed in the context of potentially targeting the intracellular stability of HIF-1α by acting on its chaperone, NQO1. This could result in novel anti-cancer therapies, always considering that the substantial genetic variability in NQO1 would likely result in different phenotypic responses among individuals.

Identification of MAGEC2/CT10 as a High Calcium-Inducible Gene in Triple-Negative Breast Cancer

The expression of the melanoma/cancer-testis antigen MAGEC2/CT10 is restricted to germline cells, but like most cancer-testis antigens, it is frequently upregulated in advanced breast tumors and other malignant tumors. However, the physiological cues that trigger the expression of this gene during malignancy remain unknown. Given that malignant breast cancer is often associated with skeletal metastasis and co-morbidities such as cancer-induced hypercalcemia, we evaluated the effect of high Ca²⁺ on the calcium-sensing receptor (CaSR) and potential mechanisms underlying the survival of triple-negative breast cancer (TNBC) cells at high Ca²⁺. We show that chronic exposure of TNBC cells to high Ca²⁺ decreased the sensitivity of CaSR to Ca²⁺ but stimulated tumor cell growth and migration. Furthermore, high extracellular Ca²⁺ also stimulated the expression of early response genes such as FOS/FOSB and a unique set of genes associated with malignant tumors, including MAGEC2. We further show that the MAGEC2 proximal promoter is Ca²⁺ inducible and that FOS/FOSB binds to this promoter in a Ca²⁺- dependent manner. Finally, downregulation of MAGEC2 strongly inhibited the growth of TNBC cells in vitro. These data suggest for the first time that MAGEC2 is a high Ca²⁺ inducible gene and that aberrant expression of MAGEC2 in malignant TNBC tissues is at least in part mediated by an increase in circulating Ca²⁺via the AP-1 transcription factor.

NQO1 Binds and Supports SIRT1 Function

Silent information regulator 2-related enzyme 1 (SIRT1) is an NAD⁺-dependent class III deacetylase and a key component of the cellular metabolic sensing pathway. The requirement of NAD⁺ for SIRT1 activity led us to assume that NQO1, an NADH oxidoreductase producing NAD⁺, regulates SIRT1 activity. We show here that SIRT1 is capable of increasing NQO1 (NAD(P)H Dehydrogenase Quinone 1) transcription and protein levels. NQO1 physically interacts with SIRT1 but not with an enzymatically dead SIRT1 H363Y mutant. The interaction of NQO1 with SIRT1 is markedly increased under mitochondrial inhibition. Interestingly, under this condition the nuclear pool of NQO1 is elevated. Depletion of NQO1 compromises the role of SIRT1 in inducing transcription of several target genes and eliminates the protective role of SIRT1 following mitochondrial inhibition. Our results suggest that SIRT1 and NQO1 form a regulatory loop where SIRT1 regulates NQO1 expression and NQO1 binds and mediates the protective role of SIRT1 during mitochondrial stress. The interplay between an NADH oxidoreductase enzyme and an NAD⁺ dependent deacetylase may act as a rheostat in sensing mitochondrial stress.

Transcription factor Ets-2 regulates the expression of key lymphotropic factors

Transcription factor Ets-2 downregulates the expression of cytokine genes and HIV-1 in resting T-cells. Herein, we studied whether Ets-2 regulates the expression of lymphotropic factors (LFs) NFAT2, NF-κΒ/p65, c-Jun, c-Fos, which regulate the activation/differentiation of T-cells, and kinase CDK10, which controls Ets-2 degradation and repression activity. In silico analysis revealed Ets-2 binding sites on the promoters of NFAT2, c-Jun, c-Fos. The T-cell lines Jurkat (models T-cell signaling/activation) and H938 (contains the HIV-1-LTR) were transfected with an Ets-2 overexpressing vector, in the presence/absence of mitogens. mRNA and protein levels were assessed by qPCR and Western immunoblotting, respectively. Ets-2 overexpression in unstimulated Jurkat increased NFAT2 and c-Jun mRNA/protein, c-Fos mRNA and NF-κΒ/p65 protein, and decreased CDK10 protein. In unstimulated H938, Ets-2 upregulated NFAT2, c-Jun and CDK10 mRNA/protein and NF-κΒ/p65 protein. In stimulated Jurkat, Ets-2 increased NFAT2, c-Jun and c-Fos mRNA/protein and decreased CDK10 mRNA/protein. In stimulated H938 Ets-2 increased NFAT2, c-Jun and c-Fos protein and reduced CDK10 protein levels. Furthermore, Ets-2 overexpression modulated the expression of pro- and anti-apoptotic genes in both cell lines. Ets-2 upregulates the expression of key LFs involved in the activation of cytokine genes or HIV-1 in T-cells, either through its physical interaction with gene promoters or through its involvement in signaling pathways that directly impact their expression. The effect of Ets-2 on CDK10 expression in H938 vs Jurkat cells dictates that, additionally to Ets-2 degradation, CDK10 may facilitate Ets-2 repression activity in cells carrying the HIV-1-LTR, contributing thus to the regulation of HIV latency in virus-infected T-cells.

FOS-ANKH and FOS-RUNX2 Fusion Genes in Osteoblastoma

BACKGROUND/AIM

Osteoblastoma is a rare benign tumor of the bones in which recurrent rearrangements of FOS have been found. Our aim was to investigate two osteoblastomas for possible genetic aberrations.

MATERIALS AND METHODS

Cytogenetic, RNA sequencing, and molecular analyses were performed.

RESULTS

A FOS-ANKH transcript was found in the first tumor, whereas a FOS-RUNX2 was detected in the second. Exon 4 of FOS fused with sequences either from intron 1 of ANKH or intron 5 of RUNX2. The fusion events introduced a stop codon and removed sequences involved in the regulation of FOS.

CONCLUSION

Rearrangements and fusions of FOS show similarities with those of HMGA2 (a feature of leiomyomas and lipomas) and CSF1 (tenosynovial giant cell tumors). The replacement of a 3'-untranslated region, controlling the gene's expression, by a new sequence is thus a common pathogenetic theme shared by FOS, HMGA2, and CSF1 in many benign connective tissue tumors.

NQO1 potentiates apoptosis evasion and upregulates XIAP via inhibiting proteasome-mediated degradation SIRT6 in hepatocellular carcinoma

Background

Our previous study has demonstrated that NAD(P)H: quinone oxidoreductase 1 (NQO1) is significantly upregulated in human liver cancer where it potentiates the apoptosis evasion of liver cancer cell. However, the underlying mechanisms of the oncogenic function of NQO1 in HCC have not been fully elucidated.

Methods

Expression of NQO1, SIRT6, AKT and X-linked inhibitor of apoptosis protein (XIAP) protein were measured by western blotting and immunohistochemistry. Additionally, the interaction between NQO1 and potential proteins were determined by immunoprecipitation assays. Furthermore, the effect of NQO1 and SIRT6 on tumor growth was determined in cell model and orthotopic tumor implantation model.

Results

We found that NQO1 overexpression in HCC enhanced SIRT6 protein stability via inhibiting ubiquitin-mediated 26S proteasome degradation. High level of SIRT6 reduced acetylation of AKT which resulted in increased phosphorylation and activity of AKT. Activated AKT subsequently phosphorylated anti-apoptotic protein XIAP at Ser87 which determined its protein stability. Reintroduction of SIRT6 or AKT efficiently rescued NQO1 knock-out-mediated inhibition of growth and induction of apoptosis. In orthotopic mouse model, NQO1 knock-out inhibited tumor growth and induced apoptosis while this effect was effectively rescued by SIRT6 overexpression or MG132 treatment partially.

Conclusions

Collectively, these results reveal an oncogenic function of NQO1 in sustaining HCC cell proliferation through SIRT6/AKT/XIAP signaling pathway.

Cancer-associated variants of human NQO1: impacts on inhibitor binding and cooperativity

Human NAD(P)H quinone oxidoreductase (DT-diaphorase, NQO1) exhibits negative cooperativity towards its potent inhibitor, dicoumarol. Here, we addressed the hypothesis that the effects of the two cancer-associated polymorphisms (p.R139W and p.P187S) may be partly mediated by their effects on inhibitor binding and negative cooperativity. Dicoumarol stabilized both variants and bound with much higher affinity for p.R139W than p.P187S. Both variants exhibited negative cooperativity towards dicoumarol; in both cases, the Hill coefficient (h) was approximately 0.5 and similar to that observed with the wild-type protein. NQO1 was also inhibited by resveratrol and by nicotinamide. Inhibition of NQO1 by resveratrol was approximately 10,000-fold less strong than that observed with the structurally similar enzyme, NRH quinine oxidoreductase 2 (NQO2). The enzyme exhibited non-cooperative behaviour towards nicotinamide, whereas resveratrol induced modest negative cooperativity (h = 0.85). Nicotinamide stabilized wild-type NQO1 and p.R139W towards thermal denaturation but had no detectable effect on p.P187S. Resveratrol destabilized the wild-type enzyme and both cancer-associated variants. Our data suggest that neither polymorphism exerts its effect by changing the enzyme’s ability to exhibit negative cooperativity towards inhibitors. However, it does demonstrate that resveratrol can inhibit NQO1 in addition to this compound’s well-documented effects on NQO2. The implications of these findings for molecular pathology are discussed.

[Ubiquitin-independent protein degradation in proteasomes]

Proteasomes are large supramolecular protein complexes present in all prokaryotic and eukaryotic cells, where they perform targeted degradation of intracellular proteins. Until recently, it was generally accepted that prior proteolytic degradation in proteasomes the proteins had to be targeted by ubiquitination: the ATP-dependent addition of (typically four sequential) residues of the low-molecular ubiquitin protein, involving the ubiquitin-activating enzyme, ubiquitin-conjugating enzyme and ubiquitin ligase. The cytoplasm and nucleoplasm proteins labeled in this way are then digested in 26S proteasomes. However, in recent years it has become increasingly clear that using this route the cell eliminates only a part of unwanted proteins. Many proteins can be cleaved by the 20S proteasome in an ATP-independent manner and without previous ubiquitination. Ubiquitin-independent protein degradation in proteasomes is a relatively new area of studies of the role of the ubiquitin-proteasome system. However, recent data obtained in this direction already correct existing concepts about proteasomal degradation of proteins and its regulation. Ubiquitin-independent proteasome degradation needs the main structural precondition in proteins: the presence of unstructured regions in the amino acid sequences that provide interaction with the proteasome. Taking into consideration that in humans almost half of all genes encode proteins that contain a certain proportion of intrinsically disordered regions, it appears that the list of proteins undergoing ubiquitin-independent degradation will demonstrate further increase. Since 26S of proteasomes account for only 30% of the total proteasome content in mammalian cells, most of the proteasomes exist in the form of 20S complexes. The latter suggests that ubiquitin-independent proteolysis performed by the 20S proteasome is a natural process of removing damaged proteins from the cell and maintaining a constant level of intrinsically disordered proteins. In this case, the functional overload of proteasomes in aging and/or other types of pathological processes, if it is not accompanied by triggering more radical mechanisms for the elimination of damaged proteins, organelles and whole cells, has the most serious consequences for the whole organism.

Specific and Fuzzy Interactions Cooperate in Modulating Protein Half-Life

Protein degradation is critical for maintaining cellular homeostasis. The 20S proteasome is selective for unfolded, extended polypeptide chains without ubiquitin tags. Sequestration of such segments by protein partners, however, may provide a regulatory mechanism. Here we used the AP-1 complex to study how c-Fos turnover is controlled by interactions with c-Jun. We show that heterodimerization with c-Jun increases c-Fos half-life. Mutations affecting specific contact sites (L165V, L172V) or charge separation (E175D, E189D, K190R) with c-Jun both modulate c-Fos turnover, proportionally to their impact on binding affinity. The fuzzy tail beyond the structured b-HLH/ZIP domain (~165 residues) also contributes to the stabilization of the AP-1 complex, removal of which decreases c-Fos half-life. Thus, protein turnover by 20S proteasome is fine-tuned by both specific and fuzzy interactions, consistently with the previously proposed "nanny" model.

The Disordered Landscape of the 20S Proteasome Substrates Reveals Tight Association with Phase Separated Granules

Proteasomal degradation is the main route of regulated proteostasis. The 20S proteasome is the core particle (CP) responsible for the catalytic activity of all proteasome complexes. Structural constraints mean that only unfolded, extended polypeptide chains may enter the catalytic core of the 20S proteasome. It has been previously shown that the 20S CP is active in degradation of certain intrinsically disordered proteins (IDP) lacking structural constrains. Here, a comprehensive analysis of the 20S CP substrates in vitro is conducted. It is revealed that the 20S CP substrates are highly disordered. However, not all the IDPs are 20S CP substrates. The group of the IDPs that are 20S CP substrates, termed 20S-IDPome are characterized by having significantly more protein binding partners, more posttranslational modification sites, and are highly enriched for RNA binding proteins. The vast majority of them are involved in splicing, mRNA processing, and translation. Remarkably, it is found that low complexity proteins with prion-like domain (PrLD), which interact with GR or PR di-peptide repeats, are the most preferential 20S CP substrates. The finding suggests roles of the 20S CP in gene transcription and formation of phase-separated granules.

Oncogenic addiction to high 26S proteasome level

Proteasomes are large intracellular complexes responsible for the degradation of cellular proteins. The altered protein homeostasis of cancer cells results in increased dependency on proteasome function. The cellular proteasome composition comprises the 20S catalytic complex that is frequently capped with the 19S regulatory particle in forming the 26S proteasome. Proteasome inhibitors target the catalytic barrel (20S) and thus this inhibition does not allow the deconvolution of the distinct roles of 20S versus 26S proteasomes in cancer progression. We examined the degree of dependency of cancer cells specifically to the level of the 26S proteasome complex. Oncogenic transformation of human and mouse immortalized cells with mutant Ras induced a strong posttranscriptional increase of the 26S proteasome subunits, giving rise to high 26S complex levels. Depletion of a single subunit of the 19S RP was sufficient to reduce the 26S proteasome level and lower the cellular 26S/20S ratio. Under this condition the viability of the Ras-transformed MCF10A cells was severely compromised. This observation led us to hypothesize that cancer cell survival is dependent on maximal utilization of its 26S proteasomes. We validated this possibility in a large number of cancer cell lines and found that partial reduction of the 26S proteasome level impairs viability in all cancer cells examined and was not correlated with cell doubling time or reduction efficiency. Interstingly, normal human fibroblasts are refractory to the same type of 26S proteasome reduction. The suppression of 26S proteasomes in cancer cells activated the UPR and caspase-3 and cells stained positive with Annexin V. In addition, suppression of the 26S proteasome resulted in cellular proteasome redistribution, cytoplasm shrinkage, and nuclear deformation, the hallmarks of apoptosis. The observed tumor cell-specific addiction to the 26S proteasome levels sets the stage for future strategies in exploiting this dependency in cancer therapy.

Small Molecule Modulation of Proteasome Assembly

The 20S proteasome is the main protease that directly targets intrinsically disordered proteins (IDPs) for proteolytic degradation. Mutations, oxidative stress, or aging can induce the buildup of IDPs resulting in incorrect signaling or aggregation, associated with the pathogenesis of many cancers and neurodegenerative diseases. Drugs that facilitate 20S-mediated proteolysis therefore have many potential therapeutic applications. We report herein the modulation of proteasome assembly by the small molecule TCH-165, resulting in an increase in 20S levels. The increase in the level of free 20S corresponds to enhanced proteolysis of IDPs, including α-synuclein, tau, ornithine decarboxylase, and c-Fos, but not structured proteins. Clearance of ubiquitinated protein was largely maintained by single capped proteasome complexes (19S-20S), but accumulation occurs when all 19S capped proteasome complexes are depleted. This study illustrates the first example of a small molecule capable of targeting disordered proteins for degradation by regulating the dynamic equilibrium between different proteasome complexes.

c-Fos mapping of brain regions activated by multi-modal and electric foot shock stress

Real-world stressors are complex and multimodal, involving physical, psychological, and social dimensions. However, the brain networks that mediate stress responses to these stimuli need to be further studied. We used c-Fos mapping in mice to characterize brain circuits activated by exposure to a single episode of multimodal stress (MMS), and compared these to circuits activated by electric foot shocks (EFS). We focused on characterizing c-Fos activity in stress-relevant brain regions including the paraventricular nucleus (PVN) of the hypothalamus and the bed nucleus of the stria terminalis (BNST). We also assessed stress-induced activation of CRH-positive neurons in each of these structures. MMS and EFS activated an overlapping network of brain regions with a similar time course. c-Fos expression within the PVN and the BNST peaked 30–60 min after exposure to both MMS and EFS, and returned to baseline levels within 24 h. Quantification of c-Fos expression within BNST subregions revealed that while c-Fos expression peaked in all subregions 30–60 min after MMS and EFS exposure, the neuronal density of c-Fos expression was significantly higher in the dorsomedial and ventral BNST relative to the dorsolateral BNST. Our preliminary assessment indicated that a great majority of MMS or EFS-activated neurons in the PVN were CRH-positive (>87%); in contrast, about 6–35% of activated neurons in the BNST were CRH-positive. Our findings indicate that both MMS and EFS are effective at activating stress-relevant brain areas and support the use of MMS as an effective approach for studying multidimensional stress in animal models. The results also reveal that the PVN and BNST are part of a common neural circuit substrate involved in neural processing related to stress.

Functional analyses of a human vascular tumor FOS variant identify a novel degradation mechanism and a link to tumorigenesis

Epithelioid hemangioma is a locally aggressive vascular neoplasm, found in bones and soft tissue, whose cause is currently unknown, but may involve oncogene activation. FOS is one of the earliest viral oncogenes to be characterized, and normal cellular FOS forms part of the activator protein 1 (AP-1) transcription factor complex, which plays a pivotal role in cell growth, differentiation, and survival as well as the DNA damage response. Despite this, a causal link between aberrant FOS function and naturally occurring tumors has not yet been established. Here, we describe a thorough molecular and biochemical analysis of a mutant FOS protein we identified in these vascular tumors. The mutant protein lacks a highly conserved helix consisting of the C-terminal four amino acids of FOS, which we show is indispensable for fast, ubiquitin-independent FOS degradation via the 20S proteasome. Our work reveals that FOS stimulates endothelial sprouting and that perturbation of normal FOS degradation could account for the abnormal vessel growth typical of epithelioid hemangioma. To the best of our knowledge, this is the first functional characterization of mutant FOS proteins found in tumors.

The NS1 protein of H5N6 feline influenza virus inhibits feline beta interferon response by preventing NF-κB and IRF3 activation

Despite the apparent lack of a feline influenza virus lineage, cats are susceptible to infection by influenza A viruses. Here, we characterized in vitro A/feline/Guangdong/1/2015, an H5N6 avian influenza virus recently isolated from cats. A/feline/Guangdong/1/2015 replicated to high titers and caused CPE in feline kidney cells. We determined that infection with A/feline/Guangdong/1/2015 did not activate the IFN-β promoter, but inhibited it by blocking the activation of NF-κB and IRF3. We also determined that the viral NS1 protein mediated the block, and that the dsRNA binding domain of NS1 was essential to perform this function. In contrast to treatment after infection, cells pretreated with IFN-β suppressed viral replication. Our findings provide an example of an H5N6 influenza virus suppressing IFN production, which might be associated with interspecies transmission of avian influenza viruses to cats.

A Signaling Network Controlling Androgenic Repression of c-Fos Protein in Prostate Adenocarcinoma Cells

The transcription factor c-Fos controls many important cellular processes, including cell growth and apoptosis. c-Fos expression is rapidly elevated in the prostate upon castration-mediated androgen withdrawal through an undefined mechanism. Here we show that androgens (5α-dihydrotestosterone and R1881) suppress c-Fos protein and mRNA expression induced by 12-O-tetradecanoylphorbol-13-acetate (TPA) or EGF in human prostate cancer (PCa) cell lines. Such suppression transpires through a transcriptional mechanism, predominantly at the proximal serum response element of the c-fos promoter. We show that androgen signaling suppresses TPA-induced c-Fos expression through repressing a PKC/MEK/ERK/ELK-1 signaling pathway. Moreover, our results support the hypothesis that p38(MAPK), PI3K, and PKCδ are involved in the androgenic regulation of c-Fos through controlling MEK/ERK. Stable silencing of c-Fos and PKCδ with shRNAs suggests that R1881 promotes cell death induced by low-dose TPA through a mechanism that is dependent on both PKCδ and loss of c-Fos expression. Reciprocally, loss of either PKCδ or c-Fos activates p38(MAPK) while suppressing the activation of ERK1/2. We also provide the first demonstration that R1881 permits cell death induced by low-dose TPA in the LNCaP androgen-dependent PCa cell line and that TPA-induced cell death is independent of exogenous androgen in the castration-resistant variants of LNCaP, C4-2 and C4-2B. Acquisition of androgen-independent killing by TPA correlates with activation of p38(MAPK), suppression of ERK1/2, and loss of c-Fos. These results provide new insights into androgenic control of c-Fos and use of PKC inhibitors in PCa therapy.

Compromising the 19S proteasome complex protects cells from reduced flux through the proteasome

Proteasomes are central regulators of protein homeostasis in eukaryotes. Proteasome function is vulnerable to environmental insults, cellular protein imbalance and targeted pharmaceuticals. Yet, mechanisms that cells deploy to counteract inhibition of this central regulator are little understood. To find such mechanisms, we reduced flux through the proteasome to the point of toxicity with specific inhibitors and performed genome-wide screens for mutations that allowed cells to survive. Counter to expectation, reducing expression of individual subunits of the proteasome's 19S regulatory complex increased survival. Strong 19S reduction was cytotoxic but modest reduction protected cells from inhibitors. Protection was accompanied by an increased ratio of 20S to 26S proteasomes, preservation of protein degradation capacity and reduced proteotoxic stress. While compromise of 19S function can have a fitness cost under basal conditions, it provided a powerful survival advantage when proteasome function was impaired. This means of rebalancing proteostasis is conserved from yeast to humans.

Activation of AHR mediates the ubiquitination and proteasome degradation of c-Fos through the induction of Ubcm4 gene expression

The ubiquitin-proteasome system (UPS) is a specific, non-lysosomal pathway responsible for the controlled degradation of abnormal and short-half-life proteins. Despite its relevance in cell homeostasis, information regarding control of the UPS component gene expression is lacking. Data from a recent study suggest that the aryl hydrocarbon receptor (AHR), a ligand-dependent transcription factor, might control the expression of several genes encoding for UPS proteins. Here, we showed that activation of AHR by TCDD and β-naphthoflavone (β-NF) results in Ubcm4 gene induction accompanied by an increase in protein levels. UbcM4 is an ubiquitin-conjugating enzyme or E2 protein that in association with ubiquitin ligase enzymes or E3 ligases promotes the ubiquitination and 26S proteasome-mediated degradation of different proteins, including p53, c-Myc, and c-Fos. We also present data demonstrating increased c-Fos ubiquitination and proteasomal degradation through the AHR-mediated induction of UbcM4 expression. The present study shows that AHR modulates the degradation of proteins involved in cell cycle control, consistent with previous reports demonstrating an essential role of the AHR in cell cycle regulation.

Proteins directly interacting with mammalian 20S proteasomal subunits and ubiquitin-independent proteasomal degradation

The mammalian 20S proteasome is a heterodimeric cylindrical complex (α7β7β7α7), composed of four rings each composed of seven different α or β subunits with broad proteolytic activity. We review the mammalian proteins shown to directly interact with specific 20S proteasomal subunits and those subjected to ubiquitin-independent proteasomal degradation (UIPD). The published reports of proteins that interact with specific proteasomal subunits, and others found on interactome databases and those that are degraded by a UIPD mechanism, overlap by only a few protein members. Therefore, systematic studies of the specificity of the interactions, the elucidation of the protein regions implicated in the interactions (that may or may not be followed by degradation) and competition experiments between proteins known to interact with the same proteasomal subunit, are needed. Those studies should provide a coherent picture of the molecular mechanisms governing the interactions of cellular proteins with proteasomal subunits, and their relevance to cell proteostasis and cell functioning.

Regulating the 20S proteasome ubiquitin-independent degradation pathway

For many years, the ubiquitin-26S proteasome degradation pathway was considered the primary route for proteasomal degradation. However, it is now becoming clear that proteins can also be targeted for degradation by the core 20S proteasome itself. Degradation by the 20S proteasome does not require ubiquitin tagging or the presence of the 19S regulatory particle; rather, it relies on the inherent structural disorder of the protein being degraded. Thus, proteins that contain unstructured regions due to oxidation, mutation, or aging, as well as naturally, intrinsically unfolded proteins, are susceptible to 20S degradation. Unlike the extensive knowledge acquired over the years concerning degradation by the 26S proteasome, relatively little is known about the means by which 20S-mediated proteolysis is controlled. Here, we describe our current understanding of the regulatory mechanisms that coordinate 20S proteasome-mediated degradation, and highlight the gaps in knowledge that remain to be bridged.

Calpain-generated natural protein fragments as short-lived substrates of the N-end rule pathway

Calpains are Ca(2+)-dependent intracellular proteases. We show here that calpain-generated natural C-terminal fragments of proteins that include G protein-coupled receptors, transmembrane ion channels, transcriptional regulators, apoptosis controllers, kinases, and phosphatases (Phe-GluN2a, Lys-Ica512, Arg-Ankrd2, Tyr-Grm1, Arg-Atp2b2, Glu-Bak, Arg-Igfbp2, Glu-IκBα, and Arg-c-Fos), are short-lived substrates of the Arg/N-end rule pathway, which targets destabilizing N-terminal residues. We also found that the identity of a fragment's N-terminal residue can change during evolution, but the residue's destabilizing activity is virtually always retained, suggesting selection pressures that favor a short half-life of the calpain-generated fragment. It is also shown that a self-cleavage of a calpain can result in an N-end rule substrate. Thus, the autoprocessing of calpains can control them by making active calpains short-lived. These and related results indicate that the Arg/N-end rule pathway mediates the remodeling of oligomeric complexes by eliminating protein fragments that are produced in these complexes through cleavages by calpains or other nonprocessive proteases. We suggest that this capability of the Arg/N-end rule pathway underlies a multitude of its previously known but mechanistically unclear functions.

The protein level of PGC-1α, a key metabolic regulator, is controlled by NADH-NQO1

PGC-1α is a key transcription coactivator regulating energy metabolism in a tissue-specific manner. PGC-1α expression is tightly regulated, it is a highly labile protein, and it interacts with various proteins--the known attributes of intrinsically disordered proteins (IDPs). In this study, we characterize PGC-1α as an IDP and demonstrate that it is susceptible to 20S proteasomal degradation by default. We further demonstrate that PGC-1α degradation is inhibited by NQO1, a 20S gatekeeper protein. NQO1 binds and protects PGC-1α from degradation in an NADH-dependent manner. Using different cellular physiological settings, we also demonstrate that NQO1-mediated PGC-1α protection plays an important role in controlling both basal and physiologically induced PGC-1α protein level and activity. Our findings link NQO1, a cellular redox sensor, to the metabolite-sensing network that tunes PGC-1α expression and activity in regulating energy metabolism.

Combination of a selective activator of the glucocorticoid receptor Compound A with a proteasome inhibitor as a novel strategy for chemotherapy of hematologic malignancies

Glucocorticoids are widely used for the treatment of hematological malignancies; however, their chronic use results in numerous metabolic side effects. Thus, the development of selective glucocorticoid receptor (GR) activators (SEGRA) with improved therapeutic index is important. GR regulates gene expression via (1) transactivation that requires GR homodimer binding to gene promoters and is linked to side effects and (2) transrepression-mediated via negative GR interaction with other transcription factors. Novel GR modulator Compound A (CpdA) prevents GR dimerization, retains glucocorticoid anti-inflammatory activity and has fewer side effects compared with glucocorticoids in vivo. Here we tested CpdA anticancer activity in human T- and B-lymphoma and multiple myeloma cells expressing GR and their counterparts with silenced GR. We found that CpdA in GR-dependent manner strongly inhibited growth and viability of human T-, B-lymphoma and multiple myeloma cells. Furthermore, primary leukemia cell cultures from T-ALL patients appeared to be equally sensitive to glucocorticoid dexamethasone and CpdA. It is known that GR expression is controlled by proteasome. We showed that pretreatment of lymphoma CEM and NCEB cells with proteasome-inhibitor Bortezomib resulted in GR accumulation and enhanced ligand properties of CpdA, shifting GR activity toward transrepression evaluated by inhibition of NFкB and AP-1 transcription factors. We also revealed remarkable GR-dependent cooperation between CpdA and Bortezomib in suppressing growth and survival of T- and B-lymphoma and multiple myeloma MM.1S cells. Overall, our data provide the rationale for novel GR-based therapy for hematological malignancies based on combination of SEGRA with proteasome inhibitors.

A mutually inhibitory feedback loop between the 20S proteasome and its regulator, NQO1

NAD(P)H:quinone-oxidoreductase-1 (NQO1) is a cytosolic enzyme that catalyzes the reduction of various quinones using flavin adenine dinucleotide (FAD) as a cofactor. NQO1 has been also shown to rescue proteins containing intrinsically unstructured domains, such as p53 and p73, from degradation by the 20S proteasome through an unknown mechanism. Here, we studied the nature of interaction between NQO1 and the 20S proteasome. Our study revealed a double negative feedback loop between NQO1 and the 20S proteasome, whereby NQO1 prevents the proteolytic activity of the 20S proteasome and the 20S proteasome degrades the apo form of NQO1. Furthermore, we demonstrate, both in vivo and in vitro, that NQO1 levels are highly dependent on FAD concentration. These observations suggest a link between 20S proteolysis and the metabolic cellular state. More generally, the results may represent a regulatory mechanism by which associated cofactors dictate the stability of proteins, thus coordinating protein levels with the metabolic status.

Stress-induced NQO1 controls stability of C/EBPα against 20S proteasomal degradation to regulate p63 expression with implications in protection against chemical-induced skin cancer

Previously, we have shown a role of cytosolic NAD(P)H:quinone oxidoreductase 1 (NQO1) in stabilization of p63 against 20S proteasomal degradation resulting in thinning of epithelium and chemical-induced skin cancer [Oncogene (2011) 30,1098–1107]. Current studies demonstrate that NQO1 control of C/EBPα against20S proteasomal degradation also contributes to the up regulation of p63 expression and protection. Western and immunohistochemistry analysis revealed that disruption of NQO1 gene in mice and mouse keratinocytes led todegradation of C/EBPα and loss of p63 gene expression. p63 promoter mutagenesis, transfection and ChIP assays identified C/EBPα binding site between nucleotide position −185 to −174 that bound to C/EBPα and up regulated p63 gene expression. Coimmunoprecipitation and immunoblot analysis demonstrated that 20S proteasomes directly interacted and degraded C/EBPα. NQO1 direct interaction with C/EBPα led to stabilization of C/EBPα against 20S proteasomal degradation. NQO1 protection of C/EBPα required binding of NADH with NQO1. Exposure of skin and keratinocytes to chemical stress agent benzo(a)pyrene led to induction of NQO1 and stabilization of C/EBPα protein resulting in an increase in p63 RNA and protein in wild type but not in NQO1−/− mice. Collectively, the current data combined with previous suggest that stress-induction of NQO1 through both stabilization of C/EBPα and increase in p63 and direct stabilization of p63 controls keratinocyte differentiation leading to protection against chemical-induced skin carcinogenesis. The studies are significant since 2–4% human individuals are homozygous and 23% are heterozygous for NQO1P187S mutation and might be susceptible to stress-induced skin diseases.

Thermo-resistant intrinsically disordered proteins are efficient 20S proteasome substrates

Based on software prediction, intrinsically disordered proteins (IDPs) are widely represented in animal cells where they play important instructive roles. Despite the predictive power of the available software programs we nevertheless need simple experimental tools to validate the predictions. IDPs were reported to be preferentially thermo-resistant and also are susceptible to degradation by the 20S proteasome. Analysis of a set of proteins revealed that thermo-resistant proteins are preferred 20S proteasome substrates. Positive correlations are evident between the percent of protein disorder and the level of thermal stability and 20S proteasomal susceptibility. The data obtained from these two assays do not fully overlap but in combination provide a more reliable experimental IDP definition. The correlation was more significant when the IUPred was used as the IDPs predicting software. We demonstrate in this work a simple experimental strategy to improve IDPs identification.

Context-dependent resistance to proteolysis of intrinsically disordered proteins

Intrinsically disordered proteins (IDPs), also known as intrinsically unstructured proteins (IUPs), lack a well-defined 3D structure in vitro and, in some cases, also in vivo. Here, we discuss the question of proteolytic sensitivity of IDPs, with a view to better explaining their in vivo characteristics. After an initial assessment of the status of IDPs in vivo, we briefly survey the intracellular proteolytic systems. Subsequently, we discuss the evidence for IDPs being inherently sensitive to proteolysis. Such sensitivity would not, however, result in enhanced degradation if the protease-sensitive sites were sequestered. Accordingly, IDP access to and degradation by the proteasome, the major proteolytic complex within eukaryotic cells, are discussed in detail. The emerging picture appears to be that IDPs are inherently sensitive to proteasomal degradation along the lines of the "degradation by default" model. However, available data sets of intracellular protein half-lives suggest that intrinsic disorder does not imply a significantly shorter half-life. We assess the power of available systemic half-life measurements, but also discuss possible mechanisms that could protect IDPs from intracellular degradation. Finally, we discuss the relevance of the proteolytic sensitivity of IDPs to their function and evolution.

E3 ligase STUB1/CHIP regulates NAD(P)H:quinone oxidoreductase 1 (NQO1) accumulation in aged brain, a process impaired in certain Alzheimer disease patients

NAD(P)H:quinone oxidoreductase 1 (NQO1) is a flavoenzyme that is important in maintaining the cellular redox state and regulating protein degradation. The NQO1 polymorphism C609T has been associated with increased susceptibility to various age-related pathologies. We show here that NQO1 protein level is regulated by the E3 ligase STUB1/CHIP (C terminus of Hsc70-interacting protein). NQO1 binds STUB1 via the Hsc70-interacting domain (tetratricopeptide repeat domain) and undergoes ubiquitination and degradation. We demonstrate here that the product of the C609T polymorphism (P187S) is a stronger STUB1 interactor with increased susceptibility to ubiquitination by the E3 ligase STUB1. Furthermore, age-dependent decrease of STUB1 correlates with increased NQO1 accumulation. Remarkably, examination of hippocampi from Alzheimer disease patients revealed that in half of the cases examined the NQO1 protein level was undetectable due to C609T polymorphism, suggesting that the age-dependent accumulation of NQO1 is impaired in certain Alzheimer disease patients.